Click the circle symbol, in properties enable the Render as Cache As Bitmap. It's a small improvement but it does actually work in HTML mode as well.

Option:

Very slight improvement (smoothing/blur).

Off:

You can see the subpixels. Turn it on, you can still see the subpixels, although slightly less:

Depending on your OS setting, you're dealing with 72DPI or 96DPI. This just isn't enough to make subpixels invisible like modern phones and tablets (300DPI+). You'll always see some level of pixellation unless you blur the heck out of the edges and then they'll seem soft.

Look at it on a device without the cacheAsBitmap and it may look much better. Try enabling it again and look at it on the same device. The DPI makes all the difference. The pixels are so small the edges won't look pixelated to the human eye.

To reinforce what sinous is saying, there isn't anything you can do on a 72 or 96 dpi screen to make the image look smoother than the "Render as Cache As Bitmap" version. The version that is rendered without "Render as Cache As Bitmap" has a minimal amount of anti-aliasing to make it look smoother. The "Render as Cache As Bitmap" has a moderate amount of anti-aliasing and more would only make the image edges look fuzzy.

Do you have example of the circle looking smoother in any other program such as photoshop on your screen?

The only thing to make the image look smoother is to render it on a higher resolution screen and use a higher resolution image.